1. Field of Invention
This invention relates generally to optical systems for the storage and retrieval of information and, more particularly, to the read/write head of the optical information storage system which directs radiation to the storage medium and then directs radiation resulting from the interaction with the medium to the radiation detectors.
2. Description of the Related Art
The optical storage systems, at present can generally be placed into one of two categories, the categories determined by the optical property used to identify different logical states on the storage medium. The first optical storage system can be referred to a differential absorption (or reflection) of a radiation beam impinging on the storage medium surface. In the differential absorption optical systems, each logical states are associated with changes in the intensity of a beam of radiation interacting with the storage medium. In the second category of optical storage systems, changes in the rotation of plane polarized beam of radiation are used to identify optical states. In either category, a multiplicity of encoding techniques can be used in the storage of the information on the storage medium.
Referring now to FIG. 1, an implementation of the differential absorption type of optical storage system, also referred to as a "write once erasable phase" change storage system, is shown. In particular, the read/write head of the optical storage system is used to determine a change in the absorption resulting from the interaction of radiation with the optical information storage medium 15. Radiation from a light source 10, typically a laser diode or light emitting diode, is collimated by lens 11. The resulting collimated light beam is passed through a polarization beam splitter 12, the beam splitter 12 positioned and oriented to pass only light having predetermined linear polarization. The linearly polarized beam is transmitted through the quarter wave plate 13. With the proper orientation of the quarter wave plate 13, the radiation beam is changed to a nearly circularly polarized state. The circularly polarized radiation beam can be considered to comprised of two components having a left hand oriented or right hand oriented component. The radiation beam is then focused by objective lens 14 on a predetermined location of storage medium 15. The storage medium 15 can be a rotating disc with storage material having at least two states of absorption. The two states of absorption will provide a detectable difference in the intensity of the radiation beam applied thereto. The radiation reflected from the storage medium 15 is recollimated by lens 14. The recollimated radiation, for which the orientation of the circular polarization is reversed by the reflection from the storage material and passage through the objective lens, is transmitted through the quarter wave plate 13 once again. The second passage of the radiation through the quarter wave plate converts the circularly polarized radiation state to a state of linear polarization of the radiation which is orthogonal relative to the linear polarization of the original radiation beam. The resulting radiation beam is applied to beam splitter 12. The polarization of the reflected radiation beam, having been rotated by 90.degree. by the media reflection and the double passage through the quarter wave plate, the reflected radiation beam will be reflected, rather than transmitted, by the beam splitter 12. The reflected radiation from the beam splitter 12 is applied to detector 18. The detector 18 responds to the magnitude of the detected radiation beam. The output signal from the photodetector 18 is a function of the amplitude of the beam reflected from the storage medium and is therefore a measure of the reflectivity of the local region upon which the radiation beam is focused. The signal from detector 18 is amplified by amplifier 20. Because there is a correlation between the local regions of controlled reflectivity with absorption, the output signal of the detector 18 identifies the reflectivity of a (currently addressed) region of the storage medium having the radiation beam applied thereto. By applying the radiation beam to localized regions of the storage medium in a manner consistent with the geometry of the information stored thereon, retrieval of information can be performed on the storage medium.
Referring to FIG. 2, the implementation of the read/write head in an optical information storage system relying on differential rotation of the planar polarization of a optical radiation caused by the interaction of the optical radiation with the storage surface is shown. This type of storage system, also known as a magneto-optical storage system, relies on the Kerr effect wherein the rotation of a plane of polarization is different when a magnetic material has a magnetic orientation parallel to or a magnetic orientation anti-parallel to the direction of the radiation interacting with the magnetic material, i.e., the differential change in polarization of a reflected beam depends upon the orientation of the magnetization of the local domain with which the radiation interacts. As with the implementation for detecting a change in reflected light amplitude, the radiation from a light source 10 is collimated by lens 11 and one plane of polarization is selected by passing the collimated beam through the partial beam splitter 12'. Because linearly polarized radiation can be considered to be comprised of two circularly polarized radiation components, the interaction with the magnetic layer forming a portion of storage medium 15 effects the two circularly polarized components differently. As a result, after interaction with the storage material, the reflected radiation is not linearly polarized parallel to the applied radiation, but an elliptical polarization of the reflected radiation results in a rotation of the reflected linear polarization due to the circular dichroism and the circular birefringence of the storage media. The reflected radiation is recollimated by objective lens 14. The recollimated beam is applied to beam splitter 12' and the components of the radiation beam orthogonal to the plane of polarization of the radiation impinging on storage medium, i.e., the components induced by the interaction, are reflected by the beam splitter 12. Some of the light with polarization parallel to the impinging radiation can also be reflected from the magneto-optic region. The radiation reflected by the beam splitter 12' is transmitted through retardation plate 16 to correct to ellipticity introduced into the radiation beam. The polarization beam splitter 17 divides the radiation reflected from beam splitter 12' into radiation components which have been rotated by the interaction with the storage material. Each detector 18 and 19 receives a component resulting from one orientation of the magnetic regions of the storage medium interacting with the impinging radiation beam. The differential amplifier 20' is used to enhance the detectability of the small signals, the rotation due to the Kerr effect typically being less than 2.degree. relative to reflected radiation which had not been subjected to differential interaction of the circularly polarized components with the optical storage material and to cancel the large DC component of the two radiation components.
The foregoing implementations of read/write heads have the problem that the retardation plates must be provided which are specific to the parameters of the device. For example, the quarter wave plate 13 (in FIG. 1) and the retardation plate 16 (in FIG. 2) both depend on the radiation emitted by the radiation source 10. The radiation source 10, e.g., a light emitting diode, can have aging effects which cause the wavelength of the emitted radiation to vary. A need has therefore been felt for a read/write head for optical storage devices which can be adjusted as conditions change and to compensate for variations in the media and for variation in the properties of the optical components. In addition, a need has been felt for a read/write head which can be adapted to either the differential absorption or the differential polarization device.